1. Field of the Invention
The present invention relates to radiation detectors and a method of making the same. More specifically, the present invention is a fundamentally new approach for growing semi-insulating CdxZn1−xTe (0#x #1) crystals with full active volume for detecting radiation in the 1 keV–5 MeV photon energy range.
2. Description of Related Art
Fundamental physical properties govern the selection of material for all radiation detector applications. Firstly, the material must exhibit high electrical resistivity. Secondly, the material must exhibit an excellent transport of the charge carriers generated by external radiation. Lastly, the material must allow an applied electric field to extend through the whole volume of the crystal, i.e., full depletion. None of these properties is exhibited in high-purity, intrinsic or undoped Cadmium Telluride, Zinc Telluride or Cadmium Zinc Telluride, i.e., CdxZn1−xTe (0≦x≦1), grown by any known method.
High-purity intrinsic CdxZn1−xTe (0≦x≦1) typically has low electrical resistivity due to the formation of a large density of intrinsic or native defects, notably cadmium (Cd) vacancies in tellurium (Te) rich growth conditions or Cd interstitials in Cd rich growth conditions. In addition, an intrinsic defect of unknown origin with a deep level at the middle of the band gap is formed in large concentrations. This intrinsic defect has electronic properties that do not permit full depletion of the device when the defect is present in large concentrations.
High resistivity CdxZn1−xTe (0≦x≦1) is typically obtained by doping with column III elements, e.g., In, Al and Ga, in a vertical or horizontal. Bridgman process or with column VII elements, e.g., Cl, in the travelling heater method. In these processes, however, significant concentrations of dopants are typically introduced that lead to diminished carrier transport and secondary effects, such as polarization of the detectors. This latter phenomenon refers to the reduction of the effective volume, i.e., efficiency, due to the collapse of the internal electric field due to carrier trapping caused by the introduced dopants or other defects. With the foregoing doping scheme it is also difficult to technologically control the achieved resistivity due to incomplete electrical compensation. As a result, electrical resistivity variation in the 1×106–1×109 Ohm-cm range is typically observed.
Electrical compensation by dopants introducing energy levels to the middle of the band gap is often used in column III-V compounds to obtain semi-insulating material. However, none of these doping schemes have solved the problem of passivating intrinsic defects forming in these materials. Examples include, but are not limited to, iron (Fe) doping in indium-phosphide (InP) and chromium (Cr) doping in gallium arsenide (GaAs). The intrinsic defect was identified as a native defect EL2 in gallium arsenide (GaAs). The addition of a single doping element with a deep level in the middle of the band gap in high concentration introduces a strong trapping of charge carriers produced by external radiation and does not passivate the intrinsic deep level that causes incomplete depletion of the devices. As a result, radiation detectors fabricated from so produced semi-insulating column III-V compounds, e.g., GaAs, are hampered by low resistivity and limited active depletion regions and do not allow for passivation of the intrinsic defects.
Incorporation of unknown impurities and the formation of native defects can render intrinsic CdxZn1−xTe (0≦x≦1) highly-resistive. However, such material typically exhibits strong carrier trapping whereupon the performance of the radiation detector is compromised. When impurities, native defects and their associations are incorporated in an uncontrolled manner the properties of the CdxZn1−xTe (0≦x≦1) ingots vary from growth to growth and exhibit strong spatial variation within individual ingots.
Accordingly, what is needed is a radiation detector made from an compound, or alloy, which has excellent carrier transport properties and which fully depletes in response to an applied electric field. What is also needed is a method of forming such an compound, or alloy.